1. Field of the Invention
This invention relates to cloud altitude measuring apparatus, and particularly to such apparatus wherein the range of the measurement is extended.
2. Prior Art
U.S. patent application Ser. No. 771,261, now U.S. Pat. No. 4,121,889, assigned to the same Assignee as the present invention, relates to a cloud altitude measuring means by which the height of a cloud is measured in steps of a certain number of meters per step. For each measuring step the signal emitter emits a series of light pulses. The signal receiver includes two integrating devices, each of which is made operative during a time interval occurring at a certain time T.sub.1 and T.sub.2, respectively, after each emitted light pulse to receive signals during the time interval. The magnitude of the signals corresponds to the magnitude of the received echo signal as well as any noise signals. Because time T.sub.1 &lt;time T.sub.2, the integrating device, which is intended for measuring the height to the cloud, receives echo signals from the lower of the two levels corresponding respectively to times T.sub.1, T.sub.2. The second integrating device, which during the measurement receives echo signals from a higher height, is employed for indicating any accompanying noise. The two signals are supplied to a summation device in which the difference, between the sum of the first signals and the sum of the second signals from a series of light pulses, is determined. When this difference exceeds a certain adjustable value, the existence of a cloud is registered. After each measuring series the integrators are set to zero and a new series of light pulses are emitted in a new measuring step.
The measurement takes place in steps up to a certain specified height, which may be, for example, 1500 m. If a cloud is present at a lower height, this is indicated, whereafter a new measurement starts from the minimum height of the measuring device. If a cloud is not indicated up to the normal measuring height, here designated N, the stepping-up is interrupted and a new measurement starts again according to the above described procedure. The normal measuring height mentioned is selected so that a safe indication is obtained if clouds exist up to that height.